In solid-state laser systems, activator/sensitizer ions present in a crystalline or glass host material or medium absorb light produced by an external pump source and thereby achieve an excited state to generate light at a known wavelength. The host laser medium is located in an optical cavity, which provides the optical feedback necessary for sustaining proper laser action.
The choice of the optical pump source to excite the laser medium strongly influences solid-state laser characteristics. Currently, semiconductor (diode) lasers are recognized as one of the most efficient pump sources to excite the laser medium. When employed as a pump source, the laser diode is used to generate light in a narrow spectral regime that overlaps the primary absorption band of the laser medium.
However, the performance of a laser diode is sensitive to the parameters such as operation temperature, driving current, optical feedback, and aging. When employed as a pump source, the fluctuations in the pump laser intensity and/or wavelength may destabilize the output power of the solid-state laser. Deviation of the operation temperature from the predetermined optimal level in just a few degrees or even less may cause significant mismatch of the pump wavelength and the gain medium absorption spectrum, resulting in dramatic drop of the DPSS laser efficiency. A further complication caused by the mismatch of the pump wavelength and the optimal absorption spectrum of the gain medium is that the unabsorbed pump light may interfere with the primary laser oscillation in the solid-state laser cavity. These problems are conventionally circumvented by maintaining the diode operation temperature precisely at a predetermined optimal level. In the prior art, optimal operation of laser diodes relies upon the inclusion of an automatic temperature controller such as thermal electric controller (TEC). One shortcoming of the automatic temperature controller is its ineffectiveness to fast fluctuations. Laser diodes that are used as pump source require excessive amounts of energy to operate. Effective heat sinks and/or water cooling are commonly employed. Consequently, the thermal control systems for pump diodes are typically large in size, complex in construction, and prone to failure.
What is more, even if the operation temperature is precisely maintained at a predetermined optimal level, the emitting wavelength of the laser diode drifts from the desired pump wavelength (optimal absorption wavelength of the laser gain medium) over time, due to the aging effect. For laser diodes that use light regulation loops for maintaining a constant laser output power, a runaway condition can occur as the diodes degrade over their lifetime. The runaway condition is most severe for pump diodes that require a high drive current and for gain media that have narrow absorption spectra. The prior art for DPSS laser monitoring and control includes U.S. Pat. No. 5,754,574, in which the diode temperature is controlled in accordance with the drive current to match the pump wavelength with the external cavity resonance; and U.S. Pat. No. 6,292,498, wherein the pump wavelength is monitored and controlled by employing a temperature modulation/detection technique. These methods are complicated and are ineffective in prevention of instable operation of the solid-state laser caused by pump light mode hopping, mode partitioning, and/or noise due to unwanted optical feedback.
Unwanted optical feedback occurs when the stray light reflected from the surfaces of the solid-state lasing gain medium and other optical elements in the laser cavity enters a photon-to-electron conversion device such as photodiode. Unwanted optical feedback may deteriorate the linear relationship between the drive current and the laser output, which has an impact on the automatic power control and even causes parasitical oscillation. To improve the linearity of the drive current vs. laser output, a method was disclosed in U.S. Pat. No. 5,856,994, wherein an index-guided type multi-transverse mode broad area laser having a single optical waveguide was used as the pump source.
One approach to stabilization of DPSS laser output relies on single longitudinal mode (SLM) operation via a wavelength selector disposed in the solid-state laser resonator. However, by absorbing a laser beam, temperature of the wavelength selector increases, which may alter the selected wavelength, causing mode hop. Global temperature control in the resonator is ineffective because the temperature rise is local, near the optical axis of the wavelength selector.
Another approach to stabilization of DPSS laser output relies on SLM pump diode. In order to maintain the laser diode in SLM and mode-hop free operation, a conventional method is based on phase coherence through, e.g., an external laser cavity in conjunction with wavelength-selective feedback from grating or externally coupled cavity. Such laser diode systems usually have relatively large sizes, high costs, less robustness, and require of sophisticated laser current and temperature control. As a matter of fact, employing a highly coherent light as pump source is not necessary or even not preferred. Another inherent drawback of the method relates to its ineffectiveness when the pass band of the wavelength selection element is broader than the interval between two adjacent wavelengths of the laser diode Fabry-Perot modes.
A more attractive strategy is based on the opposite philosophy, i.e., intentional decrease of the pump laser coherence, so that phase related optical noise can be washed out.
Apparatus and methods that employ a fiber Bragg grating (FBG) to stabilize the intensity and wavelength of a pigtailed laser diode were disclosed by a number of inventors. As described in U.S. Pat. Nos. 5,485,481, 5,715,263, 6,525,872, and 6,661,819, a fiber Bragg grating is placed in the output of the laser source with a separation sufficient to cause the laser source to operate in the “coherence collapse” regime. Consequently, the laser diode is forced to operate in multiple longitudinal modes, while the central wavelength is locked by the fiber grating to its maximum reflectivity. However, problems related to packaging and polarization significantly limit the usefulness of this method. In addition, the use of FBG for laser stabilization imposes tight manufacturing specifications on parameters such as front facet reflectivity and laser wavelength control.
A different scheme to address stabilization of laser diodes that are employed for pumping solid-state lasers or fiber amplifiers/lasers was investigated by Ziari et al. In U.S. Pat. No. 6,215,809. By the use of a dither circuit, which causes a small and continuous variation of the drive current, coherence collapse is achieved and the laser source is repeatedly perturbed from one operating mode to another at a rate that is too high for the solid-state gain element to response. In the U.S. Pat. No. 6,240,119 issued to Ventrudo, kink-free operation was achieved by repetitive switching between the states of coherence and coherence collapse through variations of the drive current amplitude at a rate considerably higher than the reciprocal of the relaxation time of the excited state of the optical gain medium, which is typically from several microseconds to milliseconds. This technique, however, is ineffective when the laser operation current is close to the critical current at which transition from coherence to coherence collapse occurs.
Stabilization of laser diode operation can also be achieved by RF modulation. With this method, the drive current changes rapidly and continuously in such a way that there is no particular longitudinal mode preferable. In other words, the laser diode operates in a multimode spectrum, normally containing a large number of longitudinal modes. Although the intensity of each individual mode fluctuates all the time, the averaged output essentially keeps unchanged and the overall optical noise decreases significantly.
There have been a number of attempts at controlling laser drive current based on high frequency, e.g. RF, modulation. In such control systems, the drive current is loaded to the high frequency signal generated by a local oscillator so that the superposed current periodically crosses the threshold level. Below this level, the laser diode is off. When the current exceeds the threshold, the laser turns on again. With repeated on-off operation at high frequency, the laser diode operates in multiple modes because there is not enough time for the completion of mode competition. As a consequence, the signal-to-noise ratio at low laser output can be improved. In fact, there have been several investigations on the application of RF stabilization to optical data reading/writing systems. Exemplary disclosures can be found in U.S. Pat. Nos. 5,175,722; 5,197,059; 5,386,409; 5,495,464; and in particular U.S. Pat. No. 6,049,073. In the last reference, laser output of approximately 20 to 100 mW has been obtained with the use of RF injection. Unfortunately, due to the sine waveform of the RF drive current, this type of stabilization schemes allows only 50% duty cycle. It is not suitable to high power laser diodes such as those used as pump sources because this would overdrive the laser and decrease its lifetime. In extreme cases, there is a possibility for the power supply to back-bias the laser diode and even destroy it.
In an attempt to extend the above-discussed RF stabilization scheme to high power region, Roddy and Markis, in U.S. Pat. Nos. 6,625,381 and 6,999,838, have invented a control system, which allows a laser diode to operate predominantly above the threshold. Specifically, the injection circuit generates a radio frequency waveform, which provides a drive current that varies between the point slightly below a lasing threshold and a level above a DC bias point. Since the drive current is asymmetric about the DC bias, a duty cycle, which is greater than 50%, can be achieved. Therefore, a high average laser output power can be obtained without the risk of exceeding the maximum rated current, Imax. Unfortunately, the RF waveforms generated from their inventive system are ineffective to certain devices, where operation modes cannot be refreshed unless the drive current periodically passes through a level that is far below the lasing threshold to completely turn off the laser and eliminate the memory in each RF cycle. Therefore, the prior art fails to stabilize lasing operation of the gain media, which employ such devices as the pump source.
Application of RF modulation to second harmonic generation (SHG) is described in, e.g., U.S. Pat. No. 6,678,306 issued to Sonoda. Dependence of the SHG efficiency on the modulation frequency and degree were investigated. Because the degree of modulation adopted by Sonoda was rather deep, the bandwidth of the primary laser spectrum was wider than the phase matching tolerance. Consequently, an external oscillator including a narrowband filter for wavelength selection was used in order to realize the quasi-phase matching of the SHG.
In another work, as disclosed in U.S. Pat. No. 6,385,219, Sonoda has investigated an index-guided laser diode, which is modulated by an electrical signal with a frequency of 20 MHz or higher and a percentage modulation between 50% and 100% (peak-to-peak normalized to two times the DC component). The laser diode emits a light with multiple transverse modes for optically pumping a solid-state laser medium. One disadvantage of the method invented by Sonoda is that optical noise increases dramatically as the percentage modulation reaches 50% or below. Another disadvantage of his invention is associated with the low duty cycle of the modulation current: increasing the percentage modulation implies increasing the peak drive current to keep the output power at the required level. When the percentage modulation is or close to 100%, the peak drive current becomes twice of the average value (the DC bias), which may cause the laser diode be deteriorated or failed. This is particularly problematic for pump diodes, normally requiring high output power. Another limitation of his invention is that the waveform of the drive current modulation may not provide the best spectral match for some gain media, in particular those with sharp absorption peak. In addition, the method disclosed by Sonoda is ineffective to gain-guided laser diodes, which normally require a higher threshold current.
There are applications, where the pump power is dynamically controlled to a level near the threshold or a coherent collapse regime. The solid-state laser operation then is very sensitive to optical noise. In the prior art, stabilization of the solid-state laser operation near the threshold is achieved by a pulse-width modulated pump source. For example, U.S. Pat. No. 7,110,167 describes an optical amplifier system, in which the pump source operates in a pulsed mode and the pulse amplitude is above the threshold current level and also above the critical current level associated with the coherent collapse regime. The pulse width is adjustable so as to produce an average power that matches a predetermined set point. One or more fiber Bragg gratings are employed for optical coupling between the pump laser diode and the optical gain medium. In addition to amplitude modulation and pulse width modulation, the laser diode can also be modulated by the repetition rate of pulses.
In spite of these efforts, a large room for further improvement of laser stabilization still remains to be filled. In particular, the stabilization methods of the prior art are subject to variations in the environmental conditions and are not applicable to certain laser diodes, normally having a high threshold, therefore, a relatively narrow range of operating current, and/or strong Amplified Spontaneous Emission (ASE), and/or mode-partition related noise. In order to refresh the laser oscillation modes in these laser diodes, the drive current must periodically drop to a level far below the threshold to completely turn off the laser operation and eliminate the memory. The successes in stabilizing DPSS lasers and fiber lasers are very limited. To date, the RF modulation, as described in the prior art, produces only broadband, multimode laser output. Because of this limitation, the prior art has not been very successful in applications requiring stable narrowband or single longitudinal mode laser sources. Examples of such applications include high-order harmonic generation, Raman scattering, and optical activation of gain media with a sharply peaked absorption spectrum. Moreover, the prior art has not disclosed the critical role of appropriately selecting the laser operation parameters. As a result, optical noise associated with mode hop and/or mode partition may still occur.
The invention disclosed in U.S. Patent Publication No. 20060215716 advantageously addresses these deficiencies and enables relatively compact and low-cost high power solid-state lasers, which can be operated stably and reliably at various wavelengths, from near infrared to the entire range of visible, in single or multiple mode(s). With one or more nonlinear optical processes, the solid-state lasers can also produce shorter wavelength lasers including UV and DUV.